Printed articles are produced on a press using printing plates or printing masters which can work using different printing techniques:                Offset printing using ink repellent and ink attracting areas on the printing master,        Flexography using compressible relief printing plates,        Gravure printing,        Silk screen printing, etc.        
For single colour (e.g. black and white) printing only a single printing master is needed.
For colour printing using more than 1 ink, a separate printing master is needed for each ink. The receiving layer passes sequentially the different printing masters on the press. A colour printing system that is often used makes overprints of yellow, magenta, cyan and black ink on paper. The four overlapping colour images combine to form a representation of the colour image.
Nowadays the printing masters are preferable fabricated using a Computer to Plate (CtP) system: the image to be reproduced is electronically provided in digital form, halftoned and is imaged directly on a printing plate precursor to obtain, after processing if required, a printing master used on a printing press.
A Ctp system comprises following a digital halftoning module and a recording apparatus.
Digital Halftoning Module
The input image delivered in an electronic form to the halftoning module is a continuous-tone image, i.e. a digital image containing pixels, which are the smallest picture elements, having multiple grey levels and/or colour levels with no perceptible color or tone quantization to them. Usually each colour component is represented by an eight bit value that can represent 256 different levels.
In a standard printing process, however, only two levels are possible corresponding to the presence of either ink or no ink. In what is called “multilevel printing systems” more than two printing grey levels are possible (for example five or seventeen), but usually this number is still not sufficient to enable the continuous tone rendering of images.
In the halftoning module the continuous-tone input image that contains a full range of tones from white through intermediate to full tone, is converted into an output image having output pixels wherein only those grey levels are present that are printable.
In the binary halftoning of black and white images, the output values correspond to either black or white, and likewise the result of binary halftoning in colour printing is either full colour or no colour.
In multilevel halftoning the continuous-tone image is converted to an image with pixels having a value out of at least 3 different levels. The pixel may be white, black or can have an intermediate grey value. Besides no ink deposition in printing, multiple levels of ink can be placed on a pixel.
A digital halftoning technique converts the multiple density values of the input pixels of a continuous tone input image into a geometric distribution of binary or multilevel halftone dots that can be printed by the reproduction device.
Each halftone dot is reproduced as a microdot or as a clustered set of microdots. A microdot is the smallest element that can be written by a reproduction device.
When the halftone dots are small enough, the eye is not capable of seeing the individual halftone dots, and only sees the corresponding spatially integrated density value of the geometric distribution.
The two main classes of halftoning techniques that are used are known as “amplitude modulation screening” (abbreviated as AM screening) and “frequency modulation screening” (abbreviated as FM screening).
According to amplitude modulation screening, the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots, the different tones of an image can be simulated. FIG. 1 shows a degrade rendered with AM screening.
According to frequency modulation screening, the distance between the fixed sized halftone dots is modulated to render different tone values. FIG. 2 shows the same degrade as in FIG. 1, but rendered with FM screening. Frequency modulation is sometimes called “stochastic screening”, because most FM screening algorithms produce halftone dot patterns that are stochastic (non-deterministic) in nature.
Three methods are widely used to produce FM screens.                A first method relies on comparing an image on a pixel-by-pixel basis with a threshold function to obtain FM-screened images. Methods to obtain such a threshold function are described in the patents U.S. Pat. No. 5,535,020, U.S. Pat. No. 5,745,259, U.S. Pat. No. 5,912,745 and U.S. Pat. No. 6,172,773 by Robert Ulichney and in the patents U.S. Pat. No. 5,543,941, U.S. Pat. No. 5,708,518 and U.S. Pat. No. 5,726,772 by Theophano Mitsa and Kevin Parker.        Lawrence Ray and James Sullivan explain a second method in WO91/12686. According to this method, continuous tone images are directly converted into frequency modulation halftones by addressing in a tone dependent way pre-calculated bitmaps that are stored in a memory.        A third method for frequency modulation was originally invented by Floyd and Steinberg and is called error diffusion. FIG. 3 explains how it works.        
The continuous tone pixel values P have a range from 0.0 (full black) to 1.0 (full white). A modified pixel value Pi of the unscreened image is compared with a fixed threshold T. If Pi is smaller than T, Hi is set to 0.0 and a black pixel is printed, else Hi is made equal to 1.0 and a white pixel is defined. The binarization of Pi introduces a quantization error Ei equal to Pi-Hi. According to the error diffusion scheme, this quantization error value is added to one or more of the unscreened pixels Pi+x,j+y, thereby generating a modified pixel value for Pi+x,j+y. Different pixels receive different fractions of the original error and this is controlled by means of “diffusion weights” c1 to cn. The sum of the diffusion weights always adds up to one. Because this scheme acts like a feedback loop, the average quantization error value converges to zero in steady state.
Robert Ulichney describes a number of enhancements over the original error diffusion algorithm in U.S. Pat. No. 4,955,065. This patent describes the use of a serpentine scan to process the input pixel values, the addition of noise on the threshold and the perturbation of the error diffusion weights to obtain more a uniform and isotropic halftone dot distribution.
Significant improvements of the original error diffusion scheme are also described in the patents U.S. Pat. No. 5,045,952 and U.S. Pat. No. 5,535,019, both by Reiner Eschbach. According to the disclosure in these patents, the threshold is modulated to either obtain an edge enhancement effect (first patent) or to improve the homogeneity of the halftone dot distributions in high and low intensity image regions (second patent). In U.S. Pat. No. 5,070,413 James Sullivan explains an improvement for screening colour images by performing error diffusion in a colorant vector space as opposed of doing scalar error diffusion for each of the colorants individually. Koen Vande Velde presented a further improvement of this idea at the International Conference on Digital Printing Technologies conference (proc. NIP17, IS&T 2001), which cal also be found in EP1 239 662. His algorithm consists of a vector error diffusion scheme in which the quantization of a colour into a set of inks is constrained by the output from an additional pre-processing step in such a way that luminance variations—and correspondingly halftoning graininess—are minimised in the final output. In U.S. Pat. No. 5,565,994 by Eschbach proposes a method that aims for a similar objective but works differently.
An improvement that is relevant with regard to our invention is also found in U.S. Pat. No. 5,087,981. In this patent Yee Ng describes the use of a printer model that takes into account halftone dot overlap to compensate for the non-linearity of printer gradation. In U.S. Pat. No. 5,854,882 by Shenge Wang, a practical method is described to characterise the dot overlap of a printer. Similar concepts regarding introducing printer models and models of the human visual system are described in the articles “Measurement of printer parameters for model based halftoning” by Thrasyvoulos N. Pappas, Chen-Koung Dong and David L. Neuhoff, published in the Journal of Electronic Imaging, July 1993 Vol. 2(3), pp 193-204. David Neuhoff patents some of the concepts presented in this article in U.S. Pat. No. 5,463,472. In U.S. Pat. No. 6,266,157, Zhigang Fan also explains a practical and efficient approach to model and calibrate the effects of dot overlap into an error diffusion scheme.
Victor Ostromoukhov points out in his presentation “A Simple and Efficient Error-Diffusion Algorithm”, published in the proceedings of the SIGGRAPH2001 conference, that more uniform halftone dot distributions are obtained at various tone values by adjusting the diffusion weights as a function of tone.
A shown in FIG. 4, in the standard FM halftoning algorithms, it is implicitly assumed that the size of the printed microdots 1 is in the same order as the size of the pixels 2 of the addressable grid of the printer and also corresponds with the size of the pixels in the original image. This assumption can create problems in printing processes where halftone dots with the size of one pixel are too small to be properly rendered. An example of such a printing process is the electrophotographic printing process. A possible solution for this problem is disclosed in U.S. Pat. No. 5,374,997 by Reiner Eschbach in which he proposes the use of an error diffusion method that makes the halftone dot n by m times larger than the size of the addressable pixels of a printer.
In one of the embodiments he explains that by means of counters the preliminary output pixels of an error diffusion process can be replicated N times horizontally and M times vertically to obtain larger halftone dots 3. The output of this scheme for the case that N=M=2 is shown in FIG. 5.
When a single colour is used in printing, the error diffusion algorithm has to be applied to the single colour.
When multicolour printing is used, each colour component needs to be processed by the halftoning algorithm.
The Recording Apparatus
The most widely used CtP system in the graphic world is a system makes use of a laser recorder for making a printing master.
Usually an infrared laser system exposes microdots corresponding to pixels on the printing plate precursor. Small spots are irradiated on the plate wherein the radiation induces chemical or structural changes within the printing plate precursor and after imaging and processing (depending upon the type of printing plate) a ready to use printing master is obtained.
In U.S. Pat. No. 6,071,369 an external drum recorder is used. Following examples are given regarding spot size etc . . . :                Infrared laser emitting at 1.06 μm with a scan speed of 17 m/s, spot size of 10 μm and energy in the plane of 248 mJ/cm2. Dwell time of the laser spot can be determined at 0.7 μs.        Infrared laser emitting at 1.06 μm with a scan speed of 2.2 m/s, spot size of 10 μm and an energy in the plane of 248 mJ/cm2. Dwell time of the laser spot can be determined at 4.8 μs.        
For each laser recording system following characteristics are important:
Dot Size
The dots that are imaged on the printing plate precursor can have different sizes. Preferably a CtP system writes microdots having a constant size. The dot size usually is in de range of 7 μm to 20 μm—depending upon the application.
Addressability
Another property of a Ctp system is the addressability. This indicates the precision by which a microdot can be positioned on the plate and is usually expressed in the number of pixels/mm or pixels/inch. All possible locations where a dot can be placed form a square (or sometimes rectangular) grid.
In laser recorders, small dots are placed within the grid to form the image to be reproduced. The grid can have a resolution up to 2400 dpi (e.g. in the AGFA Xcalibur thermal platesetter). This corresponds to an addressable pixel size of about 10 μm.
The shape and size of a laser spot in a recording system are usually designed so that the microdots can completely fill the addressable grid with, however, a minimum of overlap. Micro dot size and the size of an addressable pixel therefore are closely related to each other.
Recently also CtP processes using inkjet recording systems are suggested for preparing printing plates. An example of such a system and the elements in it is shown in FIG. 6. The printing plate precursor 4 in this example is mounted on a rotating drum 5 and an inkjet printing head 6 mounted aside the drum 5 jets ink or a reactive fluid onto the printing plate precursor 4 while the drum 5 rotates. As the drum 5 rotates, the inkjet printhead 6 slowly traverses the length of the drum 5 and a complete image is recorded pixel by pixel, line by line. Normally the inkjet printhead 6 has several nozzles 7 to jet ink so that multiple lines 8 can be recorded during a single rotation.
The inkjet printhead 6 consists of a plurality of separate tiny chambers containing ink coupled to an ink supply and each having a nozzle 7 at the end.
With thermal inkjet technology, tiny resistors rapidly heat a thin layer of liquid ink. The heated ink causes a vapour bubble to be formed, expelling or ejecting drops of ink through the nozzles 7 and placing them precisely on a surface to form text or images. As the bubble collapses, it creates a vacuum that pulls in fresh ink. This process is repeated thousands of times per second. With thermal inkjet technology, water-based inks are used. Piezoelectric inkjet technology pumps ink through nozzles 7 using pressure like in a squirt gun. A piezoelectric crystal is used to drive a very precise pump that ejects ink onto a printing medium. A wide range of ink formulations including solvent and water based and UV-curable) are compatible with the piezoelectric inkjet process.
By jetting drops of fluid, plate properties are locally influenced by chemical reaction or a printing master is formed by the properties of the image-wise applied ink itself. Some examples can be found in U.S. Pat. No. 5,275,689, U.S. Pat. No. 6,187,280, and it is also possible to form a relief printing plate directly on the plate precursor.
In US 2003/007052 a method and apparatus is described for production of lithographic printing plates using an inkjet printing system. No indication is given regarding drop volume or dot size.
The most important characteristics of an inkjet printing system for creating a printing master are:
Droplet Volume and Dot Size:
The drop jetted to the printing plate precursor has an effect on only a limited area of the plate. For known printing systems, it was measured that a droplet volume of 3 picoliter that is printed on an receiving layer in aluminium, normally results in a dot having a diameter of 20-30 μm. The resulting size of the dot on the plate is mainly affected by the surface tension effects between the liquid and the aluminium substrate.
Addressability
In modern inkjet printing systems addressability is high. Accurate positioning systems and printheads allow for the use of a grid having a resolution 115 dots/mm (2875 dpi) and more.
A example of a recording grid and printed dots in inkjet printing is shown in FIG. 7.
In this particular case, it is clear that size of the halftone dot is significantly larger than the size of an addressable pixel. This situation is different from the laser-based system.
Ctp systems using inkjet offer distinct advantages.                There is no need for processing of the printing plate with a special developer after the image is jetted onto the plate. An aqueous developer is all that is needed. No extra chemical for development of the image is needed resulting in a more ecological production method. This allows for easy and inexpensive fabrication of printing plates.        No special dark room conditions are needed. This results in shorter production times and also opens up the possibility for on-press imaging.        
Returning to the subject digital halftoning and error diffusion in particular, we have to mention that the current state of the art algorithms have a number of certain drawbacks that are of importance with regard to our invention.
Artifacts Near “Rational Tonal Values” (1/2, 2/4, 3/4, 1/9, 2/9, 3/9, etc.)
A first problem of the original error diffusion as published by Floyd and Steinberg, is that it does not behave well around the tone value of 1/2 and tone values that are multiples of 1/4 and 1/3. At and around these tone values, the standard error diffusion algorithm produces halftone dot distributions that are highly phase correlated, i.e. the dot distributions tend to be organized in locally regular, self-repeating patterns.
To explain why this problem occurs, we will concentrate first on the behavior of the Floyd and Steinberg algorithm near 50%. When Floyd and Steinberg error diffusion is performed on a tint with exactly a 50% tone value, all of the halftone dots are laid out in a checkerboard configuration. This pattern is indeed the most optimal distribution of halftone dots for this tint as it minimizes the average distance between the dots and hence also minimizes the visibility of the halftone dot pattern. For tone values just above 50% tone value, however, the algorithm will introduce an extra white pixel here and there in order to produce the correct average tone value. This extra white pixel will inevitably disturb the phase of the checkerboard pattern. FIG. 8 shows an example in which a tone value of 128/255 was rendered by means of standard Floyd and Steinberg error diffusion. These local phase shifts disturb the otherwise regular pattern, and are picked up by the eye as a disturbing artifact. A similar situation occurs for tone values just below 50%.
A similar problem also exists around the 75% tone value. At exactly 75%, Floyd and Steinberg error diffusion produces a pattern in which one out of four pixels is black and three out of four pixels are white, with all the pixels arranged in a repeating two by two matrix pattern. Just above and below this tone value, this regular pattern is disturbed by the introduction of an extra white or black pixel. An example of a 192/255 tonal value rendered with Floyd and Steinberg is shown in FIG. 9. A similar behavior is seen around a tonal value of 25% and near tonal values that are multiples of 1/9 or 1/16.
Robert Ulichney already recognized the above problems, and the method he proposes in U.S. Pat. No. 4,955,065 is effective in reducing the above undesirable artifacts. However, the use of a random element in his algorithm also introduces graininess into the image. Furthermore does his method diffuse the artifacts, rather than fundamentally suppressing them.
This statement is appreciated by comparing the halftone rendered with standard Floyd and Steinberg error diffusion and shown FIG. 8 with the halftone rendered using the improved method according to Ulichney and shown in FIG. 10.
It is an objective of the invention to avoid the introduction of objectionable artifacts in FM screening without introducing graininess in the halftoned image.
Phase Correlated Dot Positions may Introduce Low Frequency Graininess or Patterns in Color Printing
A consequence of correlated dot positions within a single separation is that it indirectly leads to phase correlation of the dot positions in the different ink separations in the case of color printing. This may introduce low frequency artifacts such as patterns and noise. Moreover, these artifacts shift and change unpredictably in the presence of misregistration between the separations.
We explain this by means of an example. Imagine a color that is printed with cyan and magenta ink separations, both having a value of 128/255. The Floyd and Steinberg algorithm produces for these tint values dot distributions that look like in FIG. 11A and FIG. 11B.
When these two separations are printed on top of each other in nearly perfect register as in FIG. 11C, they produce four possible ink overlap combinations: no ink, only cyan ink, only magenta ink, or cyan and magenta ink overprints. Because of the phase correlation of the halftone dots in the original separations, the overlap combinations themselves will also be correlated. In FIG. 11C this leads to two sorts of areas. In a first sort of area, most of the cyan and magenta dots fall on top of each other and produce a matrix of cyan and magenta overprinted dots and white spaces. In a second sort of area most of the cyan and magenta dots fall in between each other and produce a matrix of magenta and cyan dots with no or few white spaces present. Both sorts of areas produce a different color since the calorimetric addition of white and cyan on top of magenta dots does not yield exactly the same color as the calorimetric addition of cyan and magenta dots. The net result is that the color balance is not stable across the printed reproduction and that the print will appear blotchy.
When the registration between the two separation changes as in FIG. 11D, for example due to some mechanical instability of the printer or the substrate, the areas of the first kind may turn into areas of the second kind and vice versa. So not only is the color balance unstable across the print, it also varies with the registration of the printer and becomes unpredictable in the presence of misregistration. As both FIG. 11C and FIG. 11D show, the correlated artifacts in the individual separations can give yield to low frequency patterns that were not present in any of the original separations and that change as a function of the registration between the original separations.
What the above explanation shows is that correlation of the halftone dot positions can result in low frequency graininess and patterning, and in locally unstable color balance in the presence of misregistration.
The existing art uses the introduction of a random element such as the perturbation of weights or the addition of noise to the threshold in error diffusion to break up the phase correlated dot positions, but this—as was mentioned before—also introduces graininess in the halftoned image.
It is an objective of the invention to control the correlation between halftone dot distributions of the different printing masters without the introduction of graininess.
The graininess that is visible in images rendered with frequency modulation halftoning techniques is highly related to the power distribution in the two dimensional Fourier spectrum. Because the human visual system acts as a low pass filter, it is the presence of energy in the lower frequency part of the spectrum that is mostly responsible for the graininess. By filtering the power spectrum of digital halftone with the “contrast sensitivity function” of the human visual system, a measure is obtained for the amount of visible graininess. With color printing the situation becomes more complicated, as there are not one but three sensitivity functions that have to be considered, each one corresponding with one of the three sensors of the human visual system. Two articles are worth mentioning in that regard: a first one is on “The visibility of patterns in halftone images” by Huw C. Owens and Stephen Westland (Colour and Imaging Institute, Derby University, UK), and a second one is called “Contrast sensitivity for lime-cyan and purple orange gratings” by Huw C. Owens and Stephen Westland (of the Colour and Imaging Institute, Derby University, UK) Koen Vande Velde, Paul Delabastita, Juergen Jung (all of Agfa-Gevaert N. V.). Both articles were presented at the 10th conference on color imaging conference organized by the IS&T in Scottsdale. The quantitative data that is presented in this article indicates that the contrast sensitivity function (CSF) in the luminance channel of the human visual system is significantly higher than that of the in the two the channel that detects the red-green contrast and the channel that detects the yellow blue contrast. It would be advantageous to incorporate this characteristic of the human visual system when designing a digital halftoning technique for color printing.
It is the primary object of the invention to create printing masters that result in images with reduced graininess in the luminance channel of the human visual system.
The standard algorithms for error diffusion or masked based frequency modulation often result in inferior image quality. They do not take into account the special properties of the modern inkjet printing systems.
Hitherto no system exists for CtP using an inkjet printing system with a specially adapted halftoning algorithm to obtain optimum results.